The present invention relates to an active matrix type liquid crystal display apparatus, and, more particularly, to an active matrix type liquid crystal display apparatus which is suitable for an In-Plane Switching system (IPS) in which the angle of visibility is wide and power consumption is low.
In recent years, an active matrix type liquid crystal display apparatus using switching devices, typically thin film transistors, has been widely used as a monitor for a personal computer, a work station, etc., because its picture quality is substantially equal to that of a CRT, its power consumption is lower and its size is smaller than that of the CRT. In such an active matrix type liquid crystal display apparatus, an in-plane switching (IPS) system has been developed, in which the scanning wiring, the signal wiring and the common wiring are arranged on the same substrate, and two electrodes, a pixel electrode and a common electrode, are formed in the shape of a comb. Accordingly, the direction of the electric field applied to the liquid crystal is approximately parallel with the surface of the substrate. Because a liquid crystal display apparatus using the IPS system has a wider angle of visibility compared with the conventional liquid crystal display apparatus, a liquid crystal display apparatus using this system is well suitable for a direct-viewing monitor.
Although the IPS system has the characteristics described above, the aperture area through which light can be transmitted is narrow, and thus the display screen is relatively dark. Thus, it is necessary to use a bright back light in an IPS type liquid crystal display apparatus. In other words, the power consumption is very large in a liquid crystal display apparatus using the IPS system.
In order to enlarge the aperture area of a liquid crystal display apparatus using the IPS system, Japanese Patent Application Laid-Open No. 8-62578 discloses an improved display apparatus, in which the common wiring is eliminated by making the scanning wiring assume the role of the common wiring for applying a voltage from outside to a common electrode. Hereinafter, such an IPS system will be called a common-wiring-less IPS system.
In a thin film transistor used as a switching device for a normal active matrix type liquid crystal display apparatus, the gate voltage at which a current starts to flow between a drain and a source of a transistor causing the transistor to be shifted to a xe2x80x9cONxe2x80x9d state, i.e. the threshold voltage Vth of the gate, is 1 volt. In contrast, for the reason described in Japanese Patent Application Laid-open No. 8-62578, a thin film transistor with a Vth which is equal to or more than the maximum drive voltage required to optically modulate the liquid crystal, preferably equal to or more than 10 volts, is used in the common-wiring-less IPS system. It is desirable to provide uniform characteristics to such a thin film transistor, showing enhancement type switching characteristics, and further to provide a low-cost display apparatus.
The present invention provides an improved active matrix type liquid crystal display apparatus which is suitable for a low-cost common-wiring-less IPS system. Further, the present invention provides an improved thin film transistor showing switching characteristics of the uniform enhancement type.
An active matrix type liquid crystal display apparatus according to the present invention has a thin film transistor operating as a switching device. The thin film transistor has a gate insulating layer, a semiconductor layer, a drain electrode and a source electrode, and supplementary insulating layer, laminated in order on the gate electrode. The thin film transistor further has silicon oxide disposed between the gate insulating layer and the semiconductor and between the supplementary insulating layer and the semiconductor layer. Therefore, the gate insulating layer consisting of a single silicon oxide layer, or a lamination of silicon nitride and silicon oxide films, and a supplementary insulating layer consisting of silicon oxide are used in accordance with the present invention. Instead of the supplementary insulating layer consisting of silicon oxide, it is possible to form the supplementary insulating layer by oxidizing the surface of the semiconductor. Composition ratios (x) of silicon oxide (SiOx) used between the gate insulating layer and the semiconductor layer and between the supplemental insulating layer and the semiconductor layer preferably are selected so as to be equal to or more than 1.7. By manufacturing the transistor as indicated above, it becomes possible to obtain a thin film transistor which exhibits enhancement type TFT characteristics, with the threshold voltage being equal to or more than 10 volts.